This follows the vote by Iran's parliament on Sunday to further reduce ties with the U.K. because of Britain's support for tighter sanctions on Iran due to its suspected pursuit of nuclear weapons.

Iran's Press TV says "hundreds of Iranian students have pulled down the [British] flag and demanded the expulsion of the British envoy to Tehran."

The BBC says the attackers "were ... shown live on Iranian state TV throwing stones at embassy windows and breaking them. Britain's The Telegraph, which is live-blogging, reports that "petrol bombs are being thrown," but that "Iranian security forces are attempting to eject [the] protesters," according to Iran's Fars News Agency.

The only story about the incident so far at Fars' English newssite only mentions a protest "in front of the British embassy" where a British flag was burned and there were shouts of: "Down with Britain", "Down with America", "Down with Israel" and "Students are Awake and Hate Britain."

The Telegraph adds that "British officials said they were urgently trying to establish whereabouts of staff and the scope of the threat unleashed by the breach."

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